1. Field of the Invention
The present invention relates to semiconductor integrated circuits and capacitors. More particularly, the present invention relates to an apparatus formed by coupling a capacitor assembly to the underside of a semiconductor integrated circuit substrate, and then coupling the assembly formed thereby to a package.
2. The Background Art
In integrated circuit electronics, there often exists a need to use external capacitors to filter unwanted signals from conductive paths in order to ensure signal integrity at the receiving end of the conductor.
Capacitors are commonly used in filters which pass desirable frequencies and prevent the transmission of undesirable frequencies.
Filters with series capacitors are typically used to prevent low frequencies from being transmitted along a conductive path which is used to transmit desirable high frequency signals, and filters with shunt capacitors may be used to filter high frequencies from conductive paths used for supplying power to a region of circuitry. Typically, the capacitors used in these latter filters are integrated onto a semiconductor integrated circuit substrate if they have small values of capacitance, and are external to the semiconductor integrated circuit substrate if they have large values of capacitance.
FIGS. 1A and 1B are a side view and top view respectively of an integrated circuit substrate mounted on a package, with the resulting combination being mounted on a printed circuit board.
Referring to FIGS. 1A and 1B, integrated circuit 10 is shown including a multilayer package 12, an integrated circuit substrate 14, and a capacitor 16. Solder bumps 18 connect substrate 14 to package 12. Solder balls 20 connect package 12 to multilayer circuit board 22. Two conductive paths connect substrate 14 to capacitor 16, and two other conductive paths connect substrate 14 to capacitor 24.
In an integrated circuit, there are typically two types of connections made between a package and the integrated circuit substrate. First, there are input/output (I/O) connections, which typically connect the integrated circuit substrate to external environment signal sources and signal destinations. Second, there are core connections which provide connections between integrated circuit substrate circuitry and external components such as capacitors and resistors. Core connections have no external signal sources or signal destinations associated with them, and are typically centered on the integrated circuit substrate, completely surrounded by I/O connections.
Capacitors 16 and 24 are provided as examples of components designed into a system for the purpose of filtering unwanted signal frequencies from a conductive path. Although these capacitors function properly for their intended purpose, the conductive paths between the substrate and each of capacitors 16 and 24 have a characteristic high inductance which inhibits the effectiveness of the capacitors for high signal frequencies.
It is well known in the art that series inductances impede the passage of signals at higher frequencies, and series capacitances impede the passage of signals at lower frequencies. Shunt capacitors are capacitors provided between a conductive path and ground in order to shunt unwanted higher frequencies to ground. However, resistances and inductances in series with these shunt capacitors inhibits their effectiveness for this purpose.
The present invention provides a electronic assembly which provides capacitive filtering, while providing low resistance, low inductance connections between the integrated circuit substrate and the package.